Regarding whether to choose FO or FC for SIS valves
Thread Content
In SIS, the solenoid valves are usually kept powered, and the cylinders are constantly supplied with air. Does this mean that if the outcome of a certain interlock action is to close the valve, then an FC valve must be used; conversely, if the action is to open the valve, then an FO valve should be chosen?
In SIS (Safety Instrumented Systems), the selection of valves must be based on practical applications and safety requirements. Under normal circumstances, solenoid valves are usually powered, and with the cylinder continuously supplied with air, if the ultimate action of the interlock is to close the valve, a fail-close (FC) type valve is typically chosen. This is because in the event of a failure, such as a power outage or loss of control signals, the valve will close automatically to maintain the safety of the system. On the contrary, if the ultimate action of interlocking is to open a valve, valves of the Fail Open (FO) type are usually chosen. This is because in the event of a failure, such as a power outage or loss of control signals, the valve will open automatically to prevent dangerous conditions such as overpressure or overheating. However, the specific selection still requires consideration of factors such as specific process requirements, site conditions, and safety assessments. Therefore, when determining the valve selection, it is recommended to conduct a thorough safety analysis and assessment first, in order to make an appropriate choice based on the actual circumstances. .
Actually, that’s how I understand it too. Of course, the final decision still has to be based on HAZOP and LOPA analyses.
Whether it is FC or FO does not depend on the current operating conditions; rather, it depends on whether it is safe to close or open the valve in the event of a failure
This depends on the requirements of the process; for example, FC is used for the inlet and outlet isolation valves of storage tanks, while FO is used for vent valves, all with the aim of ensuring safety. However, in some special scenarios, the requirements are contradictory; for example, the emergency vent valve in a hydrogenation unit presents such a contradiction.